This invention relates to screw threaded fasteners of the kind known in the trade as inserts which are primarily intended to be pressed into objects made of synthetic resinous materials e.g. with the aid of ultrasonic vibrations applied to the fasteners. This causes the material to flow locally. A pilot bore is provided in the object, and the insert has a pilot portion at one end which is received in the bore and serves to locate and position the fastener as it begins the installation movement.
United Kingdom Patent 1279452 is a well known example of a fastener of this kind, having two axially spaced generally cylindrical bands separated from one another and from the pilot by peripheral grooves, and the two bands are each provided with helically extending teeth or ribs of sawtooth configuration which may be produced for example by knurling. The two sets are of opposite hand, i.e. one set spiral clockwise towards the pilot and the other set anticlockwise. The roots of the teeth, i.e. the minimum diameter at the base of the intertooth grooves, may be approximately equal to the diameter of the pilot and of the peripheral groove between the two bands.
During installation, the object material is displaced by the teeth and flows to fill, partly or wholly, the intertooth grooves and the peripheral grooves. This serves to provide axial and angular hold against pull-out and torque loads.
United Kingdom Patent 2039660 shows one fastener generally similar to that in United Kingdom Patent 1279452, and another which is double-ended in having pilot portion at each end. These fasteners are modified to suit insertion methods using heat as well as or instead of ultrasonic vibrations. That is to say such a fastener may be ultrasonically inserted or heat inserted.
The double-ended form is especially suitable for automatic installation from bowl feeders in which the fasteners are shaken out to process along a track to the installation point, because it avoids the necessity for orientation means to ensure that the pilot portion leads rather than trails. However it has been found experimentally that the double-ended ones will only accept lower loads. The object of the invention is to provide an improved higher load bearing double-ended fastener.